1. Field of the Invention
The present invention relates to devices that utilize evaporative, convective and/or conductive cooling to cool the human body in aid of surgery, medical treatment, therapy, or comfort. Some exemplary cooling structures include various configurations of thermal cooling devices.
2. Description of the Related Art
Temperature control in humans has important medical consequences. In order to maintain optimum health, the human body must maintain a core temperature within a very narrow range. Core body temperature changes as small as 0.1xc2x0 Celsius trigger thermoregulatory responses such as vasoconstriction, vasodilation, shivering, or sweating. A narrow temperature range is optimal for human cellular functions, biochemical reactions, and enzymatic reactions. Outside this range of temperatures, the human body experiences hypothermia (excessive cold) or hyperthermia (excessive hot).
Hyperthermia can result from illness or environmental heat stress, among other causes. In some cases, healthy people suffer hyperthermia when their natural cooling mechanisms, such as sweating, are overwhelmed during heavy physical work in a hot environment. This situation can become even worse if the person fails to drink enough fluids, and therefore cannot sweat adequately. Heat stress disorders, categorized in ascending order of severity, include: heat cramps, heat syncope, heat exhaustion, and heat stroke. Normally, discomfort causes people choose to stop working before the onset of heat exhaustion, but competitive athletics or military activities sometimes push people beyond the limits of health.
Hyperthermia can also result from fever associated with illness. Fever may arise from infection, tumor necrosis, thyroid storm, malignant hyperthermia, brain injury, and other causes. Brain injuries that cause hyperthermia usually involve the hypothalamus, and may be caused by tumors, stroke, head injury, or cardiac arrest (in the case of ischemic brain injury).
Some consequences of hyperthermial include fluid and electrolyte imbalances, increased cellular metabolic rates, and cognitive impairment. More serious consequences include motor skill impairment, loss of consciousness, and seizures. Ultimately, hyperthermia can cause irreversible cellular injury (especially of the highly metabolic brain and liver cells), organ failure, and death. Hyperthermia is a condition that, depending on its severity, may require immediate cooling treatment to return the person""s core temperature to normal.
Cooling treatment may also have other important uses. In some situations, mild or moderate hypothermia is believed to provide beneficial protection against injury. Moreover, induced hypothermia can be beneficial when the blood flow to some or all of the brain has been interrupted. Brain ischemia due to an interruption of blood flow may occur during cardiac arrest, surgery on the blood vessels of the brain, stroke, traumatic brain injury, or open heart surgery. Cooling the brain before (or in some cases after) these events can protect the brain from injury, or at least decrease the severity of the ultimate brain damage.
Physicians have used various devices and techniques to cool the human body, including pharmacological cooling and various types of mechanically induced cooling. Mechanically induced cooling approaches generally fall into one of these categories: conductive, convective, or evaporative. While different implementations have been tried, many are limited by lack of practicality, difficulty of use, ineffectiveness, and/or excessive power consumption.
One example of conductive cooling involves packing a hyperthermic person""s body in ice, or immersing the person in cool or cold water. While ice is an effective cooling agent, it is painful to the person, potentially damaging to the skin, difficult to obtain in large quantities, and impractical for long term use. Water baths can be effective, although they are not practical for the comatose or intensive care patient, or for long term use. In one less effective, but common method of conductive cooling, a person may be placed in contact with a cold-water-circulating mattress and/or cover. Water inside the mattress removes heat from the person by conduction wherever the surface of the mattress thermally contacts the person""s skin. Although there is some benefit to such devices, they are often uncomfortable and heavy, and provide inefficient thermal contact because they are not precisely shaped to the body.
In contrast to conductive cooling, convective cooling involves blowing air onto a person. Convective cooling is the least effective method of cooling from a thermodynamic point of view. Room temperature air can be blown very inexpensively with a fan. However, its cooling effectiveness is severely limited due to the thermal capacity of air, and related heat transfer coefficients.
For more efficient convective cooling, the air can be cooled before being blown onto the person. Air can be cooled, for example, with a traditional compression or heat-pump air conditioner, vortex cooling, or with thermoelectric cooling. Cooled air can also be generated using the xe2x80x9cswamp coolerxe2x80x9d principle of vaporizing water into the air stream. When water evaporates into the air, it cools the air. Then, the cooled air is applied to a person.
After the air is cooled with one of the foregoing techniques, it can be delivered to a person by cooling the air in the person""s room. To save energy, cooling can be confined to the person rather than the whole environment surrounding the person. One technique that uses this approach is the convective thermal device, which has been implemented in a variety of forms.
Although convective cooling removes the stress of environmental heat, it is minimally effective in active cooling. This limited thermodynamic effectiveness is particularly evident when trying to cool people with fevers. Generally, in order to be cooled by convection, a feverish person must be anesthetized and paralyzed to prevent the body""s heat-producing shivering response. Further, due to the thermodynamic inefficiency of convective cooling, this method of cooling uses considerable electrical power and generates considerable waste heat, which can be a problem in emergency rooms or intensive care units.
Having discussed conductive and convective cooling, the final mechanically induced cooling mechanism is evaporative cooling. Sweating is a principal example of evaporative cooling. Because water has a large heat of vaporization, large amounts of heat can be removed from the body by evaporating relatively small amounts of water. For example, when a gram of water evaporates, it extracts 540 calories of heat (2.26 kJ) from the skin. On hot summer days, many people practice basic evaporative cooling by wetting their skin or clothing with water, and permitting the water evaporate. Medical staff employ evaporative cooling by giving sponge baths to patients, where the unclothed patient is wetted with water and allowed to dry by evaporation. Sometimes a fan is pointed at the person to increase the evaporation rate. While sponge baths are indeed effective, they are labor intensive, messy, demeaning to body-conscious people, and impractical for prolonged cooling. Finally, evaporative cooing has limited effectiveness in high humidity environments.
Therefore, as shown above, conductive, convective, and evaporative cooling systems each have certain benefits and limitations. And, although some of the foregoing cooling products have certain advantages and might even enjoy some commercial success, engineers at Augustine Medical, Inc. are continually seeking to improve the performance and efficiency of human cooling systems. Some areas of possible focus include simplifying hardware designs, boosting the effectiveness of cooling systems, and cooling specific body parts.
An additional area of focus concerns the management of the liquid source during evaporative cooling. Introducing too much liquid causes liquid to spill over the area of focused cooling (the xe2x80x9ccooling fieldxe2x80x9d), and pool under the person. Pooling of contaminated liquids presents hygienic and esthetic problems in the medical environment. On the other hand, if too little liquid is supplied, the cooling field may dry out and stop or reduce cooling effectiveness.
Broadly, the present invention introduces a number of improved cooling devices that utilize evaporative and/or conductive cooling to reduce a person""s temperature in aid of surgery, medical treatment, therapy, or comfort. Some exemplary cooling structures include thermal cooling mattresses, pads, and limb-conforming sleeves.
One embodiment of the invention involves thermal cooling pads or mattresses that are placed beneath a person, and cool by conduction (and optionally evaporation, as well). According to one such embodiment, a cooling pad is embodied by a foam structure such as an open cell foam pillow or mattress. The pad defines an internal air flow passage proceeding from an air inlet to an air outlet according to a desired routing, such as a serpentine path. The pad may include various recesses shaped to receive certain body parts. The pad is wetted with an evaporation liquid, such as water. Wetting may be conducted by hand, or by routing the liquid through an internal liquid flow passage. When an air blower is coupled to the air inlet, air circulates through the air flow passage, and evaporatively cools the pad by removing warmed water vapor. Through contact with the cooled structure, the person is therefore cooled by conduction. And, depending upon whether the foam pad is covered with a sealant or not, the pad may also employ evaporative cooling if water is permitted to seep from the foam pad onto the person""s skin and subsequently evaporate.
Another type of cooling pad is a multi-layer evaporative cooling mattress. The mattress comprises an open cell foam structure having a person-receiving side and a base side. An air manifold abuts the base side. The air manifold may comprise one or more passages defined in the foam mattress, or an open mesh layer. Above the air manifold is an absorbent layer with internal liquid delivery lines. An outer film is applied to the absorbent layer.
In contrast to the cooling mattresses and pads, a different embodiment of the invention concerns a cooling sleeve. Such a device is constructed using an open cell foam structure that is configured to provide a series of multiple elongated, parallel ridges of triangular or trapezoidal cross-section. Each pair of adjacent ridges is separated by an intervening channel, and each ridge has a lateral air flow passage through it. When the first and last ridges are brought around to meet each other, the ridges form a contiguous sleeve. Within the body of the sleeve, the air flow passages cooperatively defme a continuous conduit proceeding from ridge to ridge inside the sleeve. The ridges may be interconnected, or they may be separately attached to a common base layer.
Accordingly, as discussed above, the invention may be implemented to provide various types of apparatus, such as cooling mattresses, pads, and sleeves. In contrast, certain other embodiments concern methods for utilizing such cooling equipment.
The invention affords its users with a number of distinct advantages. By using a blower to induce evaporative cooling, the invention avoids the need for power consuming refrigeration equipment. As another advantage, evaporative cooling is thermally self-limiting, because it will not produce surface temperatures that would freeze skin, as is the case with ice packs. Also, unlike ice and other phase change materials that can only maintain fixed temperatures for a limited time, cooling with this invention can be sustained indefinitely by periodically adding water to the cooling field.
As another advantage, the use of materials such as open cell foam enables the cooling devices of this invention to readily conform to the person""s body and thereby boost cooling effectiveness. This provides a marked improvement over prior approaches that practice cooling by placing an inflatable, water-filled mattress under the body. Inflatable, water-filled mattresses can only cool those body parts that support the reclining body and therefore contact the mattress. These body parts include the head, shoulder blades, buttocks, and various locations on the legs.
As another advantage, absent from previous approaches, this invention may incorporate a super-absorbent material into thermal cooling mattresses or pads. The super-absorbent material is capable of holding a large volume of water relative to its mass. This material, once wetted, can provide hours of evaporative cooling without the need for a liquid reservoir and piping system to replenish the cooling field. Consequently, cooling devices equipped with super-absorbent material discourage the introduction of too much cooling liquid, and help prevent the cooling liquid from overwhelming the cooling field and spilling over.
The invention also provides a number of other advantages and benefits, which should be apparent from the following description of the invention.